In 2016, Tomita was stabbed 61 times outside a building where she was to hold a live performance. She has since suffered from poor eyesight and post-traumatic stress disorder, according to her lawyer, Masato Takahashi.
Before the attack, Tomita told police she had received 140 comments from Tomohiro Iwazaki, including death threats, after she rejected his marriage proposal and returned his gifts including a watch and books.
Takahashi said the crime could have “easily been stopped” if Tokyo Metropolitan Police had taken sufficient action.
“There are no days since the attack that I do not think about the incident,” Tomita said in a statement, in which she called the police’s attitude “careless.”
“I will continue my fight in the hopes that this court case will help prevent potential future incidents and save potential future victims,” she added.
Tokyo Metropolitan Police said Friday it could not comment as it had not received the legal document about the case.
Iwazaki, who was aged 27 at the time of the attack, was convicted of attempted murder and sentenced in February 2017 to 14-and-a-half-years in jail.
A broader problem
After Tomita’s assault, Japan tightened its laws to classify threatening text messages and online comments as stalking — a growing problem in Japan.
In 2011, police received 14,618 complaints about stalking and made 351 arrests. In 2017, the number of complaints rose to 23,079, and arrests climbed to 926, according to official statistics.
But pop idols speaking out on the issue of stalking and assaults could raise awareness.
“If someone powerful has a hard time coming out, the average woman in Japan thinks, what chance do I have?” she said. “But idols are very relatable. This incident could help other young women to think, if she came out, then I can come out too.”
CNN’s Emiko Jozuka contributed to this report.